1. Field of the Invention
The present invention relates to an automobile antenna attachment device for attaching a telescopic automobile antenna to the body of an automobile.
2. Prior Art
FIGS. 7 through 9 show prior art devices, and in particular, FIG. 7 is a cross section of an automobile antenna attachment device, wherein reference numeral 10 is an extending and retracting type automobile antenna, and 20 is an attachment device which is used to attach the antenna 10 to the automobile body wall 30.
The antenna 10 is made up of a multi-stage extending and retracting type antenna element 11 which is formed by connecting a plurality of conductive pipes of different diameters so that the pipes are free to slide relative to each other. The antenna element 11 is held in a tube 12 so that the antenna element 11 can be freely inserted into or withdrawn from the tube 12. An insulating pipe 13 is connected to the upper end of the tube 12, and a first conductive pipe 14 is inserted into the insulating pipe 13 so that the inner surface of the pipe 14 is in electrical contact with the outer surface of the antenna element 11. A coaxial power supply cable 15 having a core conductor 15a is connected to the first conductive pipe 14. A second conductive pipe 16 provided for grounding purposes is mounted over the outer circumferences of the tube 12 and insulating pipe 13. The outer conductor of the coaxial power supply cable 15, i.e., the braided wire 15b used for shielding, is connected to this second conductive pipe 16.
The attachment device 20 includes an inner fastening assembly 20A which is installed inside of the automobile body wall 30 and an outer fastening assembly 20B which is installed outside of the automobile body wall 30. A gland washer 21 of the inner fastening assembly 20A is provided on the insulating pipe 13 so that it sits on a flange 17 formed in the top portion of the second conductive pipe 16. A gland base 22 is provided on the insulating pipe 13 so that it sits on the gland washer 21, and its upper rim portion is in contact with the inside surface of the automobile body wall 13.
As shown in FIG. 8, the gland washer 21 is an annular component (with almost no elasticity) which has a circumferential wall 21a. As seen in FIG. 9, the gland base 22 is a bowl-shaped component (with almost no elasticity), having a multiple number of projections 22a on its upper edge.
Referring back to FIG. 7, the outer fastening assembly 20B includes a pad 23 which is made of a soft plastic or soft rubber material, etc. It contacts the outside surface of the automobile body wall 30. An insulating spacer 24 of the outer fastening assembly 20B is provided on the pad 23, and a fastening nut 25 is screw-engaged with the external circumference of the upper end of the first conductive pipe 14 so that the nut 25 presses against the insulating spacer 24 and the pad 23 from above.
The upper portion of the automobile antenna 10 projects outside through an antenna attachment hole which is formed in the automobile body wall 30. The upper edge of the gland base 22 of the inner fastening assembly 20A is positioned so that it contacts the inside surface of the automobile body wall 30, and the outer fastening assembly 20B is fitted over the portion of the antenna that projects outside from the antenna attachment hole. The fastening nut 25 is then screwed onto the upper end of the first conductive pipe 14 and tightened, so that the antenna 10 is securely fastened to the automobile body wall 30.
Generally, it is extremely important that the automobile antenna be well grounded. Improper grounding must especially be avoided in the cases of ultra-short-wave antennas so as to ensure adequate transmitting and receiving. If grounding is improper, the standing-wave ratio (SWR) becomes poor, and this causes an output drop during transmission. This can also cause white noise during reception.
In the prior art antenna attachment device shown in FIGS. 7 through 9, grounding is accomplished via the inner fastening assembly 20A of the attachment device 20. Specifically, the antenna 10 is grounded via the shielding braided wire 15a of the coaxial power supply cable 15, the second conductive pipe 16, the flange 17, the gland washer 21, the gland base 22 and the automobile body wall 30. If the contact between the flange 17 of the second conductive pipe 16 and the gland washer 21, the contact between the gland washer 21 and the gland base 22, and the contact between the gland base 22 and the automobile body wall 30, are not adequately maintained, improper grounding will result. The contact pressure of the abovementioned contact areas depends primarily on the tightening force of the nut 25. Accordingly, the tightening force of the fastening nut 25 is set at a value which is great enough to ensure that the contact pressure of the respective contact areas is adequately maintained when the antenna is attached to the body of the automobile. However, when the automobile antenna 10 is used over a long period of time, the contact pressure may decrease, even if the fastening nut 25 is not loosened. Thus, improper grounding will occur and the performance of the antenna will deteriorate.
The inventors of the present invention studied this problem in various phases. As a result of their studies, the inventors ascertained that this problem is caused by permanent deformation of the pad 23. The pad 23 is installed in order to ensure that the outer fastening assembly 20B is held tightly against the outside surface of the automobile body wall 30 so that rattling will not occur, and in order to prevent rain water or other type of moisture, dirt, etc., from penetrating into the interior of the automobile body via the gap between the external fastening element 20B and the automobile body wall 30. However, as mentioned above, because this pad 23 is formed from a soft plastic, rubber, etc., the properties of the pad 23 deteriorate when it is used over a long period of time, causing a permanent deformation.